Process for the electrolytic cleaning of metals



Feb. 16, 1943.

U. C. TAINTON PROCESS FOR THE ELECTROLYTIC CLEANING 0F METALS Filed oct. 21, 1958 ATTORNE Patented Feb. 16, 1943 UNITED STATES PATENT OFFICE PROCESS FOR THE ELECTROLYTIC CLEAN- ING OF METALS Urlyn Clifton Tainton, Baltimore, Md. Application October 21, 1938, Serial No. 236,340

Claims. (CL 20d-145) My invention relates to the treatment of metal, particularly for the cleaning of the surface thereof. 'I'he invention is especially effective for removing rust, scale, carbon and slag from the surface of iron and steel, and is particularly applicable as a preliminary treatment to various coatlng operations, such as galvanizing, enameling, painting, electroplating or the like. The invention, however, as will later more fully appear, is not limited to the treatment of ferrous metal.

My invention comprises the step of subjecting the surfaces of metals to the reducing action of a metal more electropositive than the metal being treated, that is having a position in the electrochemical scale above that of the metal being treated.` For example, when iron or steel is being treated it may be subjected to the action of sodium, or calcium, which metals are considerably more electropositive than iron.

More specifically, my invention comprises the.

subjection of the metal article to be treated to the action of a more electropositive metal by a method in which the article is used first as an anode and then as a cathode in a bath comprising compounds of the morey electropositive metals, under such conditions that when the article is acting as a cathode the more electropositive metal is liberated at the surface of the article.

In presenting this invention, a specific embodiment thereof will rst be described, followed by illustrative modifications and an outline of the general principles upon which it rests.

Accordingly I shall rst describe the cleaning of steel or iron Wire as a step in the manufacture of electroplated zinc coated wire.

Referring to the drawing:

Fig. l is a diagrammatic elevation, more or less in section, of a system for cleaning and electroplating the wire; and

Fig. 2 is a plan view of the system shown in Fig. 1.

Referring rst to Figs. 1 and 2, the wire II is fed continuously from reel I2 successively through cleaning tank I3, wash tank I4, anodic cleaning tank I5, and electroplating tank I6, to reel I1. Electrodes III,4 I9 and 20 depend into tanks I3, I5 and I6 which also contain electrodes 2|, 22 and 23, respectively. The Wire runs over guide rolls 24 mounted on the tanks, and over guide rolls 25 mounted at the ends of electrodes I8, I9 and 20. The electrodes of the several tanks are connected to electric current sources 26, 21 and 28, respectively, electrodes I8, I9 and 20 being respectively negative, positive, and negative, while electrodes 2|, 22 and 23 are respectively positive, negative,

and positive. Guide rolls 25 serve to conduct current to wire II, the wire thus being rendered cathodic in tank I3, anodic in tank I5 and again cathodic in tank I6. Gas burners 29 are provided for maintaining tank I3 in a heated condition.

Tank I3 contains a fused bath of sodium hydroxide through which the wire passes. Current is passed through the fused bath to electrolyze it, the wire serving as the cathode. In this specic example of my invention, the wire passes through the bath at such a rate that each portion thereof remains immersed in the fused bath for a period of about thirty seconds and the current density employed is between and 200 amperes per square foot of cathodic surface.

From tank I 3 the wire passes through wash water in tank Id to remove the adherent sodium hydroxide, thence passing through an aqueous solution of sulphuric acid in tank I5, the concentra tion of this being 20% H2504. Here the wire acts as the anode, the current density employed being of the order of 100 amperes per square foot or greater.

The wire next passes through an aqueous solu= tion of zinc sulphate and sulphuiic acid in tank I6, the zinc sulphate being in such an amount as to give a zinc content of 7 grams per 100 cubic centimeters of solution and the sulphuric acid about 20%. Here the wire acts as the cathode, the current density being of the order of 200 amperes or greater per square foot of cathodic surface. Here the wire is electroplated with Zinc. For the anodic treatment in tank I5, the same composition may be employed as that in the coating of plating tank I6.

The anodic treatment of the Wire in tank I5 may be affected with an electrolyte of substantially the same composition as the plating electrolyte in tank L6. It is to be understood, of course, that the most "significant constituent of this electrolyte used in tank I5 is the sulphuric acid.

The treatment in tank I3 is the most signicant stage of the process so far as this particular patent application is concerned. This treatment effectively cleans the surface` of the wire, fully removing those obstacles to effective coating offerrous metal. Moreover, this treatment produces a passive condition of the surface of the metal. This passivity of the metal surface gives it a considerable( degree of resistance to corrosive iniluences.

Current densities in this sodium hydroxide cathodic cleaning operation may vary over substantial ranges. However, sumcient current density and a sumcient period of time of the metal in the cleaning bath should ordinarily be employed to effect a substantially complete reduction of surface oxides while the metal is in the bath. Ordinarily the current density should be between 25 and- 300 amperes per square foot of cathode surface. Most commonly the current is from 100 to 200 amperes per square foot 0f cathode surface.

While wire has just been given as an example of an article which may be advantageously treated by the process just described, it is evident that metal articles generally may be treated by the cathodic cleaning operation in fused sodium hydroxide whether the articles are to be fed continuously through the fused bath, as in the case of wire, or are to be treated intermittently. Obviously, the application of the cathodic, fused bath treatment is not limited to articles which are to be zinc coated as in the specific illustration just given. The method is applicable wherever it is desired to obtain an effectively cleaned metal surface. This type of cleaning is especially effective as a preparation of metal surfaces for subsequent coating operation, whether electrolytic or otherwise, including enameling and the like, but its application is as wide as the need for effectively cleaned metal surfaces.

'I'he cleaning effect produced by the process set forth above is due to the reducing action of the elemental sodium on the compounds, such as oxides. on the surface of the iron or steel being treated, the sodium being liberated in the elemental condition during electrolysis of the fused bath. The reducing action of the sodium on the iron and other oxides is enhanced by the nascent state of the electrolytically separated metal.

For the most effective results, the electrolysis of the fused sodium hydroxide should be so carried out that the elemental sodium liberated does not collect in sensible amounts, preferably not in visible amounts, upon the article being treated and which article acts as cathode during the electrolysis. To effect this result, I employ conditions during electrolysis such that the sodium as fast as it is separated from the sodium hydroxide, in the elemental state, is dissolved by or diffused into the bath of sodium hydroxide, and consequently does not gather or collect upon the surface of the article acting as cathode.

Under such conditions the sodium acts in two ways upon the oxides and other compounds on the articles surface: firstly, the sodium acts reducingly at the instant of its liberation from the sodium hydroxide, that is, it acts in its nascent condition; and secondly, the article is bathed by a solution of sodium in sodium hydroxide which acts reducingly upon any compounds on the articles surface.

The most easily imposed conditions for effecting the treatment with sodium without collecting sensible quantities of sodium on the surface of the article being treated is by control of the temperature of the fused. bath of sodium hydroxide. I maintain this fused bath at a temperature above that at which sodium collects upon the cathode. By keeping the bath at temperatures more than 20 C. in excess of the melting point of sodium hydroxide, the sodium is prevented from collecting on the cathode. At such temperatures the liberated sodium, which does not act immediately upon the compounds on the cathode surface, dissolves forthwith in the sodium hydroxide and thus the article being treated is surrounded with a highly reducing liquid which is exceedingly effective in rapidly reducing the compounds which may be upon the surface.

Preferably I employ the sodium hydroxide bath at a minimum temperature of 350 C., but, as implied above. I may go as low as a temperature -just in excess of 20v above the melting point of the sodium hydroxide. Consequently I may use a temperature as low as just in excess of 338 C. in the case of pure sodium hydroxide. Since most commercial sodium hydroxides are more or less impure, their melting points are lower than that of the pure hydroxide, ordinarily ranging from about 295 C. to 300 C., consequently with these impure sodium hydroxides I may operate at somewhat lower temperatures, if desirable, namely at temperatures which are just in excess of 315 to 320" C.

High chromium steels, including the so-called stainless steelsh such as the well known 18-8 (18% chromium, and 8% nickel), are advantageously treated by my cleaning process. In treating these high chromium steels it is usually well to employ temperatures of the order of 538 C,

In carrying out .the process above described, it may be useful in some cases to employ temperatures of the fused-bath in excess of 550 C. It may be desired, for example, to effect an annealing of wire in the fused bath at a. temperature say of 600 C. If such a temperature of the fused bath is employed in tank I3 the surface of the wire will oxidize when it enters the air.

'Ihe oxides on the surface, thus produced, how' ever, are not so closely adherent as those which normally occur on ferrous surfaces and they may easily be removed by a relatively mild pickling operation in an aqueous acid bath, such a pickling operation being much more easily and inexpensively carried out than if the metal had not previously been subjected to the fused .bath treatment. Of course, when the article, after its treatment in the sodium hydroxide bath, is given a treatment such as the anodic treatment illustrated in Figs. 1 and 2, the oxides are effectively removed.

But while it is practicable to thus remove the oxides formed on the wire or other metal I prefer in most cases to proceed in such manner as to prevent their occurrence. In the process involving the anodic treatment the removal of the oxides by the anodic operation has a disadvantage in that to effect their complete removal it is usually necessary to prolong the anodic treatment thus slowing up the entire process. Accordingly when I employ such temperatures of the fused bath as produce the tendency to ready oxidability I find 'it advantageous to use some form of procedure which will nullify this tendency.

My process is not limited in its application to sodium hydroxide. Other compounds of sodium may be used, or mixtures of different compounds. Nor is my process limited to the use of compounds of sodium. Compounds of the other alkali metals may be used, those of potassium being particularly suitable. I may also use compounds of the alkaline earth metal group including magnesium, those of calcium being economically advantageous. Hereafter, when referring generically to these various metals and their compounds, I shall usually designate them as highly electro-positive metals or compounds thereof.

Not only with sodium hydroxide but with other dioxide.

compounds of sodium as well as compounds of the other metals of the alkali metal group the metals of the alkaline earth. group. I prefer to operate at temperatures substantially above their melting points. I find that the reducing action of these highly electropositive metals is much more effective if the operating temperature of the bath is substantially in excess of the melting point of the compound or mixt` re of compounds which is used. Ordinarily the operating temperature of the bath should b`e in excess of twenty degrees above the melting point of the bath. By so proceeding, I find that I avoid the superficial reduction of compounds which may occur if lower temperatures are used, moreover the resulting reduced metal is much more easily detached than if the lower temperatures were to be used. t

By selection of the proper compound for the fused bath or by a mixture of compounds, a wide variety of effects may be produced, thus enabling one to select the conditions most suitable for his particular material and problem. For example, if lower temperatures of operation, than those securable by the use of sodium hydroxide, are desired, such temperatures can be obtained by the use of compounds of lower melting points, such, for example, as sodium nitrite, (fusing at 213 C.), either alone or mixed with sodium hydroxide. By the judicious mixing of various sodium and potassium salts, as is well known, a rather wide range of fusion points may be secured.

It will be obvious to the electro-chemist that the working conditions must frequently be altered with a change in the character of the fused bath employed. If a chloride of a highly electrol positive metal were to 'be used, for example, it would be necessary that the anodes used should be of some material resistant to the action of chlorine.

Usually in carrying out my cleaning operation it is important to maintain the fused bath relatively constant in composition or at least to allow variations only within denite limits. If there is undue variation in the composition of the fused bath the temperature will vary unduly. If, for example, sodium hydroxide is used for the bath undue formation of sodium carbonate through absorption from the air of carbon dioxide will lead to such an elevation of the melting point of the bath as to give bath temperatures too high for the most satisfactory operation of my process. Where, for instance, it is desired to operate the bath of sodium hydroxide at a temperature below 550 C. to prevent the ready oxidability of the metal, it is necessary that means be provided to prevent/ the absorption of such an amount of carbon dioxide as will raise the melting point above the desired operating temperature. The carbonate content of the fused bath will of course depend upon the rate of removal of the mixture of sodium hydroxide and sodium carbonate upon the surface of the metal being treated, the rate of replenishment of the fused bath by the addition of pure `sodium hy,- droxide, and the rate of absorption of carbon The most effective way of keeping the carbon dioxide absorption at the necessary minimum is by providing a cover for the fused bath by wiping the wire or other article as 1t issued from the bath. Sheets, for example, may be passed between rollers arranged adjacent the exit end of the bath.

My process is not limited to the treatment of ferrous metals. Articles of various metals may be eiliciently cleaned by subjecting them to the action of metals having a greater afiinity for oxygen than the metal being treated.

For example, I have effectively cleaned articles of copper, bronze, and nickel-chromium alloys by using them vas cathodes in the electrolysis of a fused bath of sodium hydroxide. It will be readily appreciated that the essential principles of this invention may be applied to metals generally, varying, as desired and as special condi-l tions demand, the details of the operation.

My process may be effectively utilized to produce sponge metal. For example, my process is very effective in treating all ferrous material which has exceedingly heavy amounts of scale. When such material is subjected to my or *.ration, as, for instance, when treated in a caustic soda bath, as above outlined, the sodium, both in the nascent form and in solution in the caustic soda, quickly and thoroughly reduces the heavy scale to sponge iron. This sponge iron may be easily removed by well known operations, such as scraping, and by the use of water sprays. The sponge iron thus removed is an important industrial product and it is frequently economically advantageous to utilize such material where heavily oxidized material is treated on a large scale.

Frequently, I find it advantageous to subject the article to be cleaned to an oxidizing treatment prior to the reducing action of the highly electropositive metal. For example, in my treatment of an article in the fused caustic soda bath, I may first pass the electric current in such direction that the article operates as an anode, thus subjecting the surface of the article to an oxidizing action, and then reverse the current to make the article a cathode and thus subjecting it to a reducing action. I'his combination treatment is particularly advantageous when the surface of the article, in addition to containing such substances as oxides and slag, also contains considerable amounts of oxidizable substances such as grease, carbon and the like. I nd this combination treatment to be especially effective in the cleaning of high chromium steels such as the well known stainless steels.

This application is in part a continuation of application Serial No. 630,233 led August 24, 1932, application Serial No. 692,378, led October '6, 1933, and of application Serial No. 128,682, filed March 2, 1937, now Patent No. 2,134,457,

dated October 25, 1938.

Iclaim:

1. The process of treating a metallic article comprising the steps of placing said article in circuit and subjecting it to electrolysis as an anode in a molten bath of a compound of a metal of the group consisting of the alkali and alkali earth metals adapted to preliminarily treat the surface with an oxidizing action preparatory to subsequent de-oxidation, continuing said anodic treatment for a time suicient to oxidize any grease, carbon or the like adhering to the said metallic article, then subjecting said article in electrical circuit to electrolysis as a solid cathode in a bath of a molten compound of a metal of the group consisting ofthe alkali and alkali earth metals adapted to be liberated at the surface oi' the cathode in said electrolysis, and subjecting the article to the action of said molten bath of the compound at a temperature below the boiling point of the liberated metal and suiliciently in excess of the fusion point of the molten compound to dissolve the liberated metal in said bathand avoid a deposit of said metal on said article, maintaining said treatment for suflicient time to completely reduce oxides, scale and the like while the article is in the bath and then delivering said treated article from said molten bath free o1 any sensible deposit of said dissolved metal.

2. The process oi.' claim 1 in which the article comprises a chromium iron alloy and the molten 15 from.

bath is a fused caustic and the electrolysis is at a current density of at least 25 amperes per square i'oot.

3. The process of claim 1 in which the article is a chromium iron alloy containing atleast 10% chromium and the electrolysis is at a current density of at least 25 amperes per square foot.

4. The process of claim 1 in which the article is a chromium iron alloy containing about 18% chromium and 8% nickel and the electrolysis is at a current density between about 100 and 200 amperes per square foot.

5. The process of claim 1 in which the treatment in the molten bath is followed by a separate treatment in a bath removing the adherent molten compound and then by a pickling treatment of the article to remove the scale there- URLYN'CLIFTON TAINTON. 

